Preparation process of enlarged latex

ABSTRACT

A process for preparing an enlarged latex, which comprises (1) causing (a) an anionic surfactant and (b) at least one surfactant selected from the group consisting of a cationic surfactant and an amphoteric surfactant to exist in a latex, (2) adding, as an aggregating and enlarging agent, at least one selected from the group consisting of (i) an inorganic acid, (ii) an organic acid, (iii) a substance which forms an acid in water, (iv) a combination of at least two substances which are reacted with each other to form an acid, and (v) a substance which forms an acid by exposure to active rays to the latex in the presence of these surfactants, and (3) causing the acid derived from the aggregating and enlarging agent to act on the latex, thereby enlarging the particle diameter of the latex, an enlarged latex obtained by the preparation process, a graft copolymer obtained by polymerizing a polymerizable monomer with the enlarged latex, and a resin composition comprising the graft copolymer and a thermoplastic resin.

TECHNICAL FIELD

The present invention relates to a process for preparing an enlargedlatex having a larger particle diameter by enlarging a latex byaggregation, and more particularly to a process for preparing anenlarged latex, which is economical and high in productivity and permitsenlarging a latex while retaining the stability of the latex. Thepresent invention relates to a process for preparing a graft copolymermaking use of such an enlarged latex, a resin composition containing thegraft copolymer and a thermoplastic resin, etc.

BACKGROUND ART

A latex is an emulsion in which a polymer such as rubber or a plastic isdispersed in the form of colloid in water by an emulsifying agent. Assynthetic latices, are prepared, for example, rubber latices such asstyrene-butadiene rubber latices, acrylonitrile-butadiene rubber laticesand polychloroprene rubber latices; and resin latices such as vinylacetate (co)polymer latices, styrene (co)polymer latices and acrylicester (co)polymer latices by emulsion polymerization. The latices areused in a wide variety of fields such as a field of synthetic resins, afield of paints, a field of treating agents of paper and fabrics and afield of civil engineering such as concrete and asphalt.

A latex is generally an emulsion in which a polymer having a fineparticle diameter is dispersed. However, a latex having a largerparticle diameter is required in many application fields as necessaryfor the end application intended. A graft copolymer obtained bypolymerizing a vinyl monomer in the presence of a rubber latex is usedas an impact modifier or the like for thermoplastic resins. In suchgraft copolymers, those having various particle diameters are chosen foruse as necessary for the end application intended. In order to obtain agraft copolymer having a large particle diameter, a latex having a largeparticle diameter is preferably used.

A latex is generally prepared by an emulsion polymerization process.However, it takes a long polymerization time to obtain a latex having alarge particle diameter by a seed polymerization process, so thatproductivity is lowered. In particular, when a diene monomer or amonomer mixture containing a diene monomer and a vinyl monomer will bepolymerized by the seed polymerization process to obtain a latex havinga large particle diameter, it takes a very long polymerization time. Asa process for obtaining a latex having a large particle diameter in ashort period of time, there has heretofore been proposed a process inwhich an inorganic acid, an organic acid, a salt, a polymeric flocculantand/or a latex for enlargement is added to a latex having a smallparticle diameter to enlarge the latex by aggregation.

For example, Japanese Patent Application Laid-Open No. 71603/1997 hasproposed a process in which so mild shear as to produce no rubber lumpis applied to a diene polymer rubber latex obtained by emulsionpolymerization to enlarge the latex by aggregation mainly by virtue ofBrownian aggregation while facilitating mixing of a flocculant. TheBrownian aggregation means that latex particles undergo Brownianmovement, thereby colliding with one another to aggregate. Morespecifically, the process comprises lowering the number of revolutionsupon stirring in enlargement of the latex by aggregation making use ofthe flocculent to conduct aggregation mainly by virtue of Brownianmovement, thereby inhibiting the formation of aggregated lumps toenlarge the latex by aggregation. This process uses shear aggregation bya stirring blade and Brownian aggregation in combination. According tothis process, particles of the latex can be enlarged to a certainextent. However, the particles cannot be enlarged to a sufficient extentjudging from Examples thereof. In addition, in this process, an acid isused as a flocculant, and the flocculant is added so as to keep the pHof the system at 5 or lower. When it is intended to raise the pH of thesystem by adding a basic substance to the enlarged latex obtained bythis process so as to stabilize the latex, however, the stability of thelatex becomes insufficient due to a high salt concentration in thesystem. Therefore, when graft polymerization of a vinyl monomer has beenattempted in the presence of said latex, a problem that deposit is easyto be formed has arisen.

Japanese Patent Publication No. 2229/1969 has proposed a process inwhich a formaldehydesulfoxylate and a peroxide are added to an aqueousdispersion containing fine particles of a butadiene polymer to enlargethe particles while causing graft polymerization to progress by adding amonomer to the dispersion. According to this process, however, it isdifficult to enlarge the particles to a sufficient extent.

Japanese Patent Application Laid-Open No. 45921/1981 has proposed aprocess in which an acrylic ester-unsaturated acid copolymer latexobtained by polymerization in the presence of an anionic surfactant andhaving a pH of 4 or higher is added to a synthetic rubber latex the pHof which has been adjusted to 7 or higher, thereby enlarging theparticle diameter of the synthetic rubber latex. According to thisprocess, however, it is necessary to separately prepare the latex forenlargement. Therefore, this process is complicated in operation and notpreferable from the economical point of view. In addition, when graftpolymerization of a vinyl monomer has been attempted in the presence ofthe synthetic rubber latex enlarged by adding such an acrylicester-unsaturated acid copolymer latex, a problem that the stability ofthe latex is impaired to form aggregated lumps has arisen.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a process forpreparing an enlarged latex, which is economical and high inproductivity and permits enlarging a latex while retaining the stabilityof the latex.

Another object of the present invention is to provide a process forpreparing an enlarged latex the stability of which is not impaired evenwhen a polymerizable monomer is graft polymerized in the presence of theenlarged latex.

A further object of the present invention is to provide an enlargedlatex having such excellent various properties, a process for preparinga graft copolymer by polymerizing a polymerizable monomer in thepresence of such an enlarged latex, a resin composition containing thegraft copolymer and a thermoplastic resin, and a graft copolymer (latex,slurry or particles) having an even particle diameter distribution.

The present inventors have carried out an extensive investigation with aview toward achieving the above objects. As a result, it has beenconceived to cause an anionic surfactant and a cationic surfactantand/or an amphoteric surfactant to exist in a latex and add an acid or asubstance capable of forming an acid as an aggregating and enlargingagent thereto to cause the acid to act, thereby enlarging the particlediameter of the latex.

When the pH of a latex stabilized by an anionic surfactant is lowered byan acid, the primary particles of the latex are aggregated and enlarged.However, the stabilizing effect by the anionic surfactant becomes weakas the pH is lowered, so that the stability of the latex tends to beimpaired. On the other hand, when an anionic surfactant and a cationicsurfactant and/or an amphoteric surfactant are caused to exist in alatex, (1) the system is stable by virtue of the stabilizing effect bythe anionic surfactant when the pH is high, (2) the stabilizing effectby the anionic surfactant is weakened when the pH is lowered by an acid,so that aggregation and enlargement of latex particles occur, and (3)the system is stabilized again by virtue of the stabilizing effect bythe cationic surfactant and/or the amphoteric surfactant when the pH ismore lowered. As a result, a stabilized enlarged latex can be obtained.Even when the stabilized enlarged latex is used in a graftpolymerization reaction, the stability of the system is not impaired.

When the lowering of the pH in this process is conducted by using acombination of at least two substances which are reacted with each otherto form an acid, such as a combination of hydrogen peroxide with aformaldehydesulfoxylate, and enlargement by aggregation is conducted byBrownian aggregation without conducting stirring during the enlargementby aggregation, an enlarged latex having a narrow particle diameterdistribution and a large particle diameter can be stably formed.

A graft copolymer obtained by graft polymerizing a polymerizable monomeron the enlarged latex obtained by the process according to the presentinvention can be used in various application fields by itself and alsoexhibits excellent properties as an impact modifier for thermoplasticresins. The present invention has been led to completion on the basis ofthese findings.

According to the present invention, there is thus provided a process forpreparing an enlarged latex by enlarging a latex by aggregation, whichcomprises

(1) causing

(a) an anionic surfactant and

(b) at least one surfactant selected from the group consisting of acationic surfactant and an amphoteric surfactant

to exist in the latex,

(2) adding, as an aggregating and enlarging agent, at least one selectedfrom the group consisting of:

(i) an inorganic acid,

(ii) an organic acid,

(iii) a substance which forms an acid in water,

(iv) a combination of at least two substances which are reacted witheach other to form an acid, and

(v) a substance which forms an acid by exposure to active rays

to the latex in the presence of these surfactants, and

(3) causing the acid derived from the aggregating and enlarging agent toact on the latex, thereby enlarging the particle diameter of the latex.

According to the present invention, there is also provided an enlargedlatex obtained by the preparation process described above. According tothe present invention, there are further provided a process forpreparing a graft copolymer, which comprises polymerizing apolymerizable monomer in the presence of the enlarged latex describedabove, and a resin composition comprising the graft copolymer obtainedby this preparation process and a thermoplastic resin.

According to the present invention, there is still further provided agraft copolymer containing an anionic surfactant and at least onesurfactant selected from the group consisting of a cationic surfactantand an amphoteric surfactant and having a ratio (Dv/Dn) of the volumeaverage particle diameter (Dv) to the number average particle diameter(Dn) of 1.2 to 1.8.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Latex:

A latex used in the present invention may be that prepared by anyprocess. However, a latex synthesized by emulsion polymerization isgenerally used. No particular limitation is imposed on componentsforming the latex. As examples thereof, however, may be mentioned(co)polymers of diene monomers such as butadiene, isoprene andchloroprene; (co)polymers of vinyl monomers such as styrene,acrylonitrile, acrylic esters, methacrylic esters, ethylene, vinylchloride, vinylidene chloride, vinyl acetate and vinyl fluoride;copolymers of a diene monomer with a vinyl monomer; silicone resins suchas polyorganosiloxanes; and polyester, epoxy resins, melamine resins,polyamide and polyurethane. These polymers may be used either singly orin any combination thereof. The latex may have a structure such as acore/shell structure and be an organic.inorganic composite latex.

In order to regulate a surface charge of the latex to control theenlargement by aggregation, a latex obtained by copolymerizing such adiene monomer and/or vinyl monomer as described above with a monomerhaving an anionic and/or cationic functional group may also be used.Examples of the monomer having the functional group include acrylicacid, methacrylic acid, itaconic acid, fumaric acid, acrylamide,methacrylamide, hydroxyethyl methacrylate, hydroxyethyl acrylate andglycidyl methacrylate. Further, a crosslinkable monomer such asdivinylbenzene, ethylene glycol dimethacrylate, trimethylolpropanetrimethacrylate or 1,3-butanediol diacrylate may be used in combination.

Upon polymerization of the latex, a radical polymerization initiator,heat decomposable polymerization initiator, redox initiator or the likemay be used. Besides, light, X-rays or the like may also be used. Achain transfer agent such as t-dodecylmercaptan, n-octylmercaptan orα-methylstyrene dimer may be used upon the polymerization as needed.Surfactants are used upon the polymerization. Regarding this, however,description will be given subsequently.

These latices may be used either singly or in any combination thereof.Of these, the (co)polymer latices of diene monomers, (co)polymer laticesof vinyl monomers, and copolymer latices of a diene monomer with a vinylmonomer are preferred. When the process according to the presentinvention is applied to a diene (co)polymer latex containing a dienemonomer such as butadiene, a great effect is achieved, and particularlypreferable results are yields from the viewpoint of shortening ofpolymerization time.

Examples of the diene (co)polymer latex include (co)polymer latices ofdiene monomers such as butadiene, isoprene and chloroprene; andcopolymer latices of a diene monomer with a vinyl monomer. Examples ofthe vinyl monomer copolymerized with the diene monomer include aromaticvinyl monomers such as styrene and α-methylstyrene; alkyl (meth)acrylatemonomers such as methyl methacrylate and n-butyl acrylate; andunsaturated nitrile monomers such as acrylonitrile. Example of the(co)polymer latices of the diene monomers include polybutadiene latices.Examples of the diene copolymer include copolymer rubbers of a dienemonomer such as butadiene with a vinyl monomer such as styrene. Noparticular limitation is imposed on the copolymerization ratio of thediene monomer to the vinyl monomer. However, for example, copolymers of50 to 99 wt. % of a diene monomer with 1 to 50 wt. % of a vinyl monomermay be mentioned.

No particular limitation is imposed on the particle diameter (volumeaverage particle diameter) of the latex. However, it is preferably 200nm or smaller, more preferably 150 nm or smaller because even a latexhaving a fine particle diameter of 150 nm or smaller, furthermore 100 nmor smaller can be effectively enlarged by the process according to thepresent invention.

2. Surfactant:

In the present invention, (a) an anionic surfactant and (b) at least onesurfactant selected from the group consisting of a cationic surfactantand an amphoteric surfactant are caused to exist in the latex. As anexample of a process for causing these surfactants to exist in thelatex, is mentioned a process in which emulsion polymerization isconducted with the anionic surfactant upon preparation of the latex, andthe cationic surfactant and/or the amphoteric surfactant is then addedto the resultant latex. When the cationic surfactant and/or theamphoteric surfactant is added to the latex containing the anionicsurfactant, however, deposit may be formed in some cases. In such acase, is preferred a process in which the cationic surfactant and/or theamphoteric surfactant has been added in advance together with theanionic surfactant upon the preparation of the latex. In order toprevent the formation of the deposit, a process in which polymerizationis conducted with the anionic surfactant, and the cationic surfactantand/or the amphoteric surfactant is added during or after thepolymerization in a state that the cationic surfactant and/or theamphoteric surfactant has been mixed with an excess amount of theanionic surfactant may also be adopted.

As examples of the anionic surfactant, may be mentioned those commonlyused in emulsion polymerization, such as carboxylic acid salts, sulfonicacid salts, sulfuric ester salts and phosphoric ester salts. Of these,are preferred the carboxylate type surfactants, for example, alkalimetal salts of higher fatty acids, such as sodium oleate, potassiumoleate, sodium stearate, potassium stearate, sodium myristate, potassiummyristate, sodium palmitate, potassium palmitate, potassium laurate,potassium undecanate, sodium linolate, potassium linolate, potassiumcaprylate, potassium nonanate and potassium caprinate; alkali metalsalts of rosinic acid, such as disproportionating potassium rosinate;alkali metal salts of alkylsarcosinic acids; and alkali metal salts ofalkenylsuccinic acids. These anionic surfactants may be used eithersingly or in any combination. When a carboxylate type anionic surfactantis used, a sulfonate type anionic surfactant and a nonionic surfactantmay be used as assistant in combination.

Examples of the cationic surfactant include quaternary ammonium saltshaving at least one alkyl group, primary to tertiary amine salts havingat least one alkyl group, alkylphosphonium salts and alkylsulfoniumsalts. More specifically, as examples thereof, may be mentionedbenzalkonium chloride, alkyltrimethylammonium chlorides, alkylamineacetates, alkylamine hydrochlorides, dialkyldimethylammonium chloride,alkylisoquinolinium chlorides and alkylisoquinolinium bromides.

As examples of the amphoteric surfactant, may be mentioned betaines suchas N-octylbetaine, N-decylbetaine, N-undecylbetaine, N-dodecylbetaine,N-tetradecylbetaine, N-hexadecylbetaine, octylbetaine, decylbetaine anddodecylbetaine; carboxylic acid type amphoteric surfactants containing abetaine such as sulfobetaine or sulfatebetaine; sulfate type amphotericsurfactants, such as hydroxyethylimidazoline sulfate; and sulfonic acidtype amphoteric surfactants such as imidazolinesulfonic acid.

The surfactants selected from the group consisting of the cationicsurfactants and amphoteric surfactants may be used either singly or inany combination thereof. No particular limitation is imposed on theusing ratio of (a) the anionic surfactant to (b) the cationic surfactantand/or the amphoteric surfactant. It is however desirable that (b) thecationic surfactant and/or the amphoteric surfactant be caused to existin a proportion of preferably 0.01 to 100 mol, more preferably 0.1 to 80mol, still more preferably 1 to 50 mol per 100 mol of (a) the anionicsurfactant from the viewpoints of stability of the latex, control ofenlargement by aggregation and stability of the enlarged latex. Thesurfactants are generally used in a proportion of 0.1 to 5 parts byweight in total per 100 parts by weight of the (co)polymer component inthe latex or the monomer(s) forming the (co)polymer component.

3. Aggregating and Enlarging Agent:

In the present invention, at least one selected from the groupconsisting of (i) an inorganic acid, (ii) an organic acid, (iii) asubstance which forms an acid in water, (iv) a combination of at leasttwo substances which are reacted with each other to form an acid, and(v) a substance which forms an acid by exposure to active rays is usedas an aggregating and enlarging agent.

As examples of the inorganic acid, may be mentioned hydrochloric acid,sulfuric acid, nitric acid and phosphoric acid. As examples of theorganic acid, may be mentioned acetic acid, formic acid, tartaric acid,malic acid and butyric acid. As examples of the substance which forms anacid in water, may be mentioned acid anhydrides such as acetic anhydrideand maleic anhydride; and esters such as sulfuric esters and phosphoricesters.

As the combination of at least two substances which are reacted witheach other to form an acid, is preferred a combination of substanceswhich form an acid by a redox reaction, and specific examples thereofmay include combinations of peroxide/formaldehyde, peroxide/sulfoxylicacid salt and peroxide/formaldehydesulfoxylate. Of these, thecombination of peroxide/formaldehydesulfoxylate (for example, sodiumformaldehydesulfoxylate) is preferred.

No particular limitation is imposed on the substance which forms an acidby exposure to active rays so far as it is a substance which forms aBrφnsted acid or Lewis acid by exposure to active rays. Specificexamples thereof include onium salts, halogenated organic compounds,quinonediazide compounds, α,α-bis(sulfonyl)diazomethane compounds,α-carbonyl-α-sulfonyl-diazomethane compounds, sulfonic compounds,organic ester compounds, organic acid amide compounds and organic acidimide compounds. Examples of the active rays include ultraviolet rays,far ultraviolet rays, electron rays and laser beams.

These acids and acid-forming substances are generally added in the formof an aqueous solution to the latex. With respect to the amount added,it is desired that an amount in which the enlargement of the latex iseasy to be achieved within limits not forming any aggregated lump of thelatex be confirmed by experiments, since the acidity varies according tothe kind of the acid or acid-forming substance. Preferable experimentalexamples in this respect are specifically shown in respective Examples.

In order to adjust the particle diameter of the enlarged latex, theaggregating and enlarging agent and a salt may be used in combination.The salt may be contained in the latex in advance or added prior to anenlarging treatment by aggregation. Examples of salts having nopH-buffering effect include sodium chloride, potassium chloride andcalcium chloride. Examples of salts having a pH-buffering effect includesodium pyrophosphate, sodium carbonate and ammonium sulfate.

4. Enlarging Treatment:

In the present invention, the acid derived from the aggregating andenlarging agent is caused to act on a latex, thereby enlarging theparticle diameter of the latex. More specifically, an anionic surfactantand a cationic surfactant and/or an amphoteric surfactant are caused toexist in the latex, and an acid or acid-forming substance is added asthe aggregating and enlarging agent to the latex, thereby causing theacid derived from the aggregating and enlarging agent to act on thelatex. When the aggregating and enlarging agent is an inorganic acid, anorganic acid or a substance which forms an acid in water, the acidimmediately performs an aggregating and enlarging action in the latex.

When the aggregating and enlarging agent is a combination of at leasttwo substances which are reacted with each other to form an acid, achemical reaction between these substance takes place in the latex toform an acid, and said acid performs an aggregating and enlargingaction. When the aggregating and enlarging agent is a substance whichforms an acid by exposure to active rays, the latex is exposed to theactive rays to form an acid, and the acid thus formed performs anaggregating and enlarging action.

In order to enhance the enlarging effect upon the enlarging treatment byaggregation, ultrasonic vibration may be applied. No particularlimitation is imposed on the treating temperature upon the enlargementby aggregation. However, it is preferably 20 to 90° C. that is atemperature generally easy to control, more preferably a temperature notlower than the glass transition temperature of the polymer componentforming the latex.

The enlarging treatment by aggregation may be conducted while stirringthe latex. However, the stirring may be stopped after the aggregatingand enlarging agent is added, and stirring and mixing are lightlyconducted so as to uniformly disperse. When no stirring of the latex isconducted, the enlargement of the latex takes place by Brownianaggregation of latex particles. When the aggregating and enlarging agentis the combination of at least two substances which are reacted witheach other to form an acid, or the substance which forms an acid byexposure to active rays in particular, a method in which the latex isenlarged by Brownian aggregation without conducting stirring of thelatex in the step of causing the acid to act on the latex is preferredfrom the viewpoint of providing an enlarged latex narrow in particlediameter distribution represented by a ratio (Dv/Dn) of the volumeaverage particle diameter (Dv) to the number average particle diameter(Dn). In this case, it is considered that the formation of the acid inthe system and Brownian aggregation by the action of the acid are causedto progress in well balanced relation, and consequently a latex having anarrow particle diameter distribution and uniformly enlarged is formed.The latex having a narrow particle diameter distribution and uniformlyenlarged can exhibit high quality and high performance when it is usedin various fields.

After the enlarging treatment of the latex, a basic substance such assodium hydroxide, potassium hydroxide, sodium carbonate or potassiumcarbonate is generally added to the latex to neutralize the acid. Thesebasis substances are generally added in the form of an aqueous solutionto the latex.

No particular limitation is imposed on the particle diameter of theenlarged latex. However, it is generally 150 nm or greater, preferablyabout 200 to 1,000 nm in terms of the volume average particle diameter(Dv) thereof. Even when the process according to the present inventionis applied to a latex having a fine particle diameter, the latex can beenlarged to a volume average particle diameter of preferably 200 nm orgreater, more preferably 250 nm or greater. The latex may be enlarged to300 nm or greater, furthermore 350 nm or greater as needed. When thevolume average particle diameter is enlarged to 300 nm or greater inparticular, the effect of the present invention is markedly exhibited.No particular limitation is imposed on the particle diameterdistribution (Dv/Dn) of the enlarged latex. According to the process ofthe present invention, however, an enlarged latex having an evenparticle diameter of preferably 1.2 to 1.8, more preferably 1.2 to 1.6,still more preferably 1.2 to 1.5 in terms of Dv/Dn can be provided.

5. Graft Polymerization and Graft Copolymer:

The enlarged latex obtained by the process described above is subjectedto graft polymerization, whereby a graft copolymer can be obtained. Thegraft polymerization can be conducted by polymerizing a polymerizablemonomer in the presence of the enlarged latex. No particular limitationis imposed to a graft polymerization process. However, an emulsionpolymerization process and a suspension polymerization process arepreferred. Upon the graft polymerization, a surfactant such as ananionic surfactant, cationic surfactant, amphoteric surfactant ornonionic surfactant; a suspending agent such as an organic suspendingagent or inorganic suspending agent; etc. may be suitably added to morestabilize the system. No particular limitation is imposed on thepolymerizable monomer used in the graft polymerization. However, it ispreferably a vinyl monomer. No particular limitation is also imposed onthe weight ratio of the enlarged latex to the polymerizable monomer.However, they are preferably graft polymerized in such proportions thatthe solid content of the enlarged latex is 5 to 95 wt. %, and thecontent of the vinyl monomer is 95 to 5 wt. %.

As examples of the vinyl monomer, may be mentioned aromatic vinylmonomers such as styrene and α-methylstyrene; aromatic polycyclic vinylmonomers such as 4-vinylbiphenyl and 2-vinylnaphthalene; unsaturatednitriles such as acrylonitrile and methacrylonitrile; alkyl(meth)acrylate monomers such as methyl methacrylate,and butyl acrylate;unsaturated carboxylic acids such as acrylic acid, methacrylic acid,maleic acid and maleic anhydride; and maleimide monomers such asmaleimide and N-phenylmaleimide. These vinyl monomers may be used eithersingly or in any combination thereof.

A polyfunctional vinyl monomer such as divinylbenzene, allylmethacrylate, ethylene glycol dimethacrylate or 1,3-butylenedimethacrylate may be suitable used in combination upon thepolymerization of the vinyl monomer. A chain transfer agent such ast-dodecylmercaptan or n-octylmercaptan may also be used.

The vinyl monomer graft polymerized on the enlarged latex may be addedinto the reaction system at once, in several portions, continuously orin any combination thereof. When the graft polymerization is conductedin two or more stages, the monomer compositions in respective stages maybe the same or different from each other.

When the graft polymerization is conducted with the enlarged latex byemulsion polymerization or suspension polymerization, a graft copolymerlatex or slurry containing the anionic surfactant and the cationicsurfactant and/or the amphoteric surfactant is provided. The ratio(Dv/Dn) of the volume average particle diameter (Dv) to the numberaverage particle diameter (Dn) in the graft copolymer according to thepresent invention is preferably 1.2 to 1.8, more preferably 1.2 to 1.6,still more preferably 1.2 to 1.5, and so the particle diameter thereofis even. In a graft copolymer obtained by conducting the graftpolymerization by suspension polymerization, however, the particlediameter distribution of a portion to be grafted does often notcorrespond to the particle diameter distribution of the graft copolymer.Therefore, the particle diameter distribution (Dv/Dn) of such a graftcopolymer is determined to mean the ratio of the volume average particlediameter to the number average particle diameter (Dn) in the portion tobe grafted.

Since the number of coarse particles becomes decreased by the narrowparticle diameter distribution of the graft copolymer and deteriorationof transparency caused by scattering of light due to the coarse particlecan be prevented, the transparency thereof can be enhanced. In addition,since particles having a particle diameter effective for enhancement ofstrength become increased by the narrow particle diameter distributionof the graft copolymer, the strength thereof is also enhanced. It ispreferred from the viewpoint of transparency in particular that thekinds and combination of the vinyl monomers subjected to graftpolymerization be selected in such a manner that the refractive indicesof the polymer (enlarged latex particles) to be grafted and theresulting graft copolymer consist with each other. For example, when theenlarged latex is a butadiene rubber latex or styrene-butadiene rubberlatex, styrene, methacrylic acid, butyl acrylate, etc. are suitablycombined as vinyl monomers subjected to the graft polymerization,whereby the refractive indices of the polymer to be grafted and theresulting graft copolymer can be caused to consist with each other. Itis preferred that a difference in refractive index between the polymerto be grafted and the graft copolymer be 0.02 or smaller.

The volume average particle diameter of the graft copolymer is generally150 nm or greater, preferably about 200 to 1,000 nm. When the volumeaverage particle diameter is 300 nm or greater in particular, the effectby the application of the present invention is marked. As describedabove, however, the volume average particle diameter of a graftcopolymer obtained by conducting the graft polymerization by thesuspension polymerization is determined to mean the volume averageparticle diameter of the portion to be grafted.

After the graft polymerization, the graft copolymer is provided as alatex, slurry or particles separated and collected therefrom. Noparticular limitation is imposed on a method for separating andcollecting the graft copolymer as particles from the latex or slurry.However, as examples thereof, may be mentioned a method in which acoagulant such as hydrochloric acid or calcium chloride is added to thelatex to coagulate particles, and the resultant slurry is dehydrated anddried, and a method in which the latex is sprayed in heated air to dryit. In any method, additives such as antioxidants, ultravioletabsorbents, anti-blocking agents, pigments, fillers, lubricants,antistatic agents and antibacterial agents may be suitably added beforeor after the coagulation and drying. The graft copolymer may be used asa thermoplastic resin by itself. In such a case, dry particles may besubjected to molding or forming and processing as they are or afterpelletizing them. No particular limitation is imposed on a forming ormolding method. For example, processing methods used for ordinarythermoplastic resins, such as calendering, extrusion, blow molding andinjection molding, may be adopted.

6. Resin Composition:

The graft copolymer according to the present invention can be blendedwith a thermoplastic resin to prepare a resin composition. A blendingratio (based on solids content) between both may be suitably determinedaccording to the purpose of use and desired properties. In general, theymay be suitably selected within ranges of 0.1 to 99.9 wt. % for thegraft copolymer and 99.9 to 0.1 wt. % for the thermoplastic resin. Inmany cases, good results can be yielded in proportions of 1 to 99 wt. %for the graft copolymer and 99 to 1 wt. % for the thermoplastic resin.When the graft copolymer is blended as an impact modifier with athermoplastic resin such as a vinyl chloride resin, they may be oftenblended with each other in proportions of 1 to 50 wt. % for the graftcopolymer and 99 to 50 wt. % for the thermoplastic resin.

No particular limitation is imposed on the thermoplastic resin. Asexamples thereof, however, may be mentioned polystyrene, high impactpolystyrene (HI polystyrene resins), acrylic resins, methylmethacrylate-styrene resins (MS resins), vinyl chloride resins,chlorinated vinyl chloride resins, acrylonitrile-styrene resins (ASresins), acrylonitrile-butadiene-styrene resins (ABS resins),thermoplastic polyester resins and polycarbonate resins. Thesethermoplastic resins may be used either singly or in any combinationthereof.

Additives, for example, antioxidants, ultraviolet absorbents,anti-blocking agents, pigments, fillers, lubricants, antistatic agents,antibacterial agents, etc. may be suitably added upon the preparation ofthe resin composition. No particular limitation is imposed on a blendingmethod. Therefore, they may also be mixed with each other by means of aribbon blender, Henschel mixer or the like. This resin composition maybe subjected to molding or forming and processing as it is or afterpelletizing it. No particular limitation is imposed on a forming ormolding method. For example, processing methods used for ordinarythermoplastic resins, such as calendering, extrusion, blow molding andinjection molding, may be adopted.

When the resin composition comprising the graft copolymer and thethermoplastic resin is used as a transparent molded or formed product,it is desirable that the respective compositions be adjusted so as tolessen a difference in refractive index between the graft copolymer andthe thermoplastic resin in addition to the lessened difference inrefractive index between the polymer to be grafted and the graftcopolymer. When transparency is regarded as important, it is preferablethat the difference in refractive index between them be controlled to0.02 or smaller.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples. However, the presentinvention is not limited at all by these examples. Incidentally, alldesignations of “part” or “parts” and “%” as will be used in thefollowing examples mean part or parts by weight and wt. % unlessexpressly noted. Physical properties in the examples were determined inaccordance with the following respective methods.

(1) Volume Average Particle Diameter and Particle Diameter Distribution:

The volume average particle diameter (Dv; also referred to as “averageparticle diameter” merely) of each sample was determined by subjectingan electron microscope photograph obtained by using a transmission typeelectron microscope to image analysis by an image analyzer (manufacturedby Asahi Chemical Industry Co., Ltd.; IP-500PC). The particle diameterdistribution (Dv/Dn) thereof was determined by calculating out a ratioof the volume average particle diameter (Dv) to a number averageparticle diameter (Dn) obtained by subjecting the sample to imageanalysis in the same manner as described above.

(2) Refractive Index of Enlarged Latex-Forming Component:

A cast film was formed with each enlarged latex sample, and theresultant cast film was immersed in methyl alcohol and then vacuum driedat room temperature for 24 hours, thereby preparing a sample film. Withrespect to the sample film, the refractive index was measured at 23° C.by means of an Abbe's refractometer.

(3) Refractive Indices of Graft Copolymer and Thermoplastic Resin:

Each graft copolymer or thermoplastic resin sample was hot pressed at200° C. to prepare a sample film. With respect to the sample film, therefractive index was measured at 23° C. by means of an Abbe'srefractometer.

(4) Impact Strength of Molded Product:

Specimen having a thickness of 3 mm or 6 mm were prepared by means of aninjection molding machine IS-80 manufacture by Toshiba Machine Co., Ltd.With respect to these specimens, the impact strength was determined at23° C. or −10° C. in accordance with JIS K 7110.

(5) Transparency of Molded Product:

Each graft copolymer or thermoplastic resin composition sample waspelletized by means of an extruder and a pelletizer. The resultantpellet sample was hot pressed at 200° C. to prepare a sample platehaving a thickness of 3 mm. With respect to this sample plate, thetransmittance for parallel rays and haze were measured at 23° C. bymeans of a haze meter.

Example 1

1. Preparation of Latex:

After a pressure container equipped with a stirrer was charged with

sodium chloride 0.075 parts ferrous sulfate 0.005 parts disodiumethylenediaminetetraacetate 0.008 parts sodium formaldehydesulfoxylate0.05 parts benzalkonium chloride (cationic 0.02 parts surfactant)potassium oleate (anionic surfactant) 0.37 parts distilled water 200parts and purged with nitrogen, diisopropylbenzene hydroperoxide 0.1parts butadiene 75 parts styrene 25 parts

were added. The contents were then held at 60° C. for 15 hours toconduct polymerization, thereby obtaining Latex (A-1) having aconversion of 98% and a volume average particle diameter of 98 nm.

2. Enlargement by Aggregation:

While holding the Latex (A-1) obtained above at 70° C.,

sodium formaldehydesulfoxylate 3.8 parts (5% aqueous solution) hydrogenperoxide (5% aqueous solution) 2.08 parts

were added, and the contents were stirred and mixed. The stirring wasthen stopped to hold the mixture for 1 hour. Five parts of sodiumhydroxide (1% aqueous solution) were then added, and the contents werestirred and mixed to obtain Enlarged Latex (B-1). Enlarged Latex (B-1)had a volume average particle diameter of 460 nm and a Dv/Dn of 1.38 andwas mechanically stable. Incidentally, the sum total of deposits andattachments to the polymerizer was 0.05% based on the charged monomers.

Example 2

1. Preparation of Latex:

Latex (A-2) having a conversion of 98% and a volume average particlediameter of 92 nm was obtained by conducting polymerization in the samemanner as in Example 1 except that 100 parts of butadiene were used inplace of 75 parts of butadiene and 25 parts of styrene in thepreparation of the latex in Example 1.

2. Enlargement by Aggregation:

Latex (A-2) obtained above was enlarged by aggregation in the samemanner as in Example 1 except that

sodium formaldehydesulfoxylate 4.2 parts (5% aqueous solution) hydrogenperoxide (5% aqueous solution) 2.32 parts

were added to Latex (A-2), thereby obtaining Enlarged Latex (B-2).Enlarged Latex (B-2) had a volume average particle diameter of 500 nmand a Dv/Dn of 1.35 and was mechanically stable. Incidentally, the sumtotal of deposits and attachments to the polymerizer was 0.07% based onthe charged monomers.

Example 3

1. Preparation of Latex:

Latex (A-3) having a conversion of 98% and a volume average particlediameter of 98 nm was obtained by conducting polymerization in the samemanner as in Example 1 except that the amount of benzalkonium chloride(cationic surfactant) was increased from 0.02 parts to 0.05 parts in thepreparation of the latex in Example 1, and 100 parts of butadiene wereused in place of 75 parts of butadiene and 25% of styrene.

2. Enlargement by Aggregation:

Latex (A-3) obtained above was enlarged by aggregation in the samemanner as in Example 1 except that

sodium formaldehydesulfoxylate 3.5 parts (5% aqueous solution) hydrogenperoxide (5% aqueous solution) 3.5 parts

were added to Latex (A-3), thereby obtaining Enlarged Latex (B-3).Enlarged Latex (B-3) had a volume average particle diameter of 250 nmand a Dv/Dn of 1.40 and was mechanically stable. Incidentally, the sumtotal of deposits and attachments to the polymerizer was 0.04% based onthe charged monomers.

Comparative Example 1

It was intended to obtain an enlarged latex in the same manner as inExample 1 except that no benzalkonium chloride (cationic surfactant) wasadded in the preparation of the latex in Example 1. Although an enlargedlatex having an average particle diameter of 500 nm was formed afterholding the contents for 1 hour after the stirring was stopped in thestep of enlarging by aggregation, solids were deposited as lumps as soonas stirring was started after sodium hydroxide for neutralization wasadded, so that no stable enlarged latex was able to be obtained.

Comparative Example 2

Latex (A-1) having a volume average particle diameter of 98 nm obtainedin Example 1 was used as seed particles to attempt the preparation of alatex having a great particle diameter by a seed polymerization processin place of the enlargement by aggregation.

After a polymerizer was charged with

sodium chloride 0.075 parts ferrous sulfate 0.005 parts disodiumethylenediaminetetraacetate 0.008 parts sodium formaldehydesulfoxylate0.05 parts potassium oleate 0.37 parts Latex (A-1) (in terms of solids)1 parts distilled water 200 parts and purged with nitrogen, the contentswere held at 60° C., and butadiene 74.25 parts styrene 24.75 partsdiisopropylbenzene hydroperoxide 1 part sodium formaldehydesulfoxylate0.5 parts

were then added over 60 hours. The contents were then held at 60° C. for30 hours. As a result, the volume average particle diameter was enlargedto 430 nm, but the conversion did not reach 95%.

Comparative Example 3

1. Preparation of Latex:

After a pressure container equipped with a stirrer was charged with

tetrasodium pyrophosphate 0.1 parts ferrous sulfate 0.005 parts disodiumethylenediaminetetraacetate 0.008 parts sodium formaldehydesulfoxylate0.05 parts potassium oleate 0.37 parts distilled water 200 parts andpurged with nitrogen, diisopropylbenzene hydroperoxide 0.1 partsbutadiene 75 parts styrene 25 parts

were added. The contents were then held at 60° C. for 15 hours to obtainLatex (a-3) having a conversion of 98% and a volume average particlediameter of 97 nm.

2. Enlargement by Aggregation:

While holding the Latex (a-3) obtained above at 60° C., 0.2 parts ofdisodium dodecyl phenyl ether disulfonate were added, and the number ofrevolutions upon stirring was then decreased. Although 1.2 parts ofphosphoric acid (5% aqueous solution) were added to conduct enlargementby aggregation, the latex was solidified.

<Consideration>

Example 1 shows an example where the present invention was applied tothe butadiene-styrene copolymer latex, and Examples 2 and 3 showexamples where the present invention was applied to the butadienepolymer latices. In any case, a stable enlarged latex was obtained in ashort period of time.

On the other hand, Comparative Example 1 shows an example where nocationic surfactant was used upon the preparation of thebutadiene-styrene copolymer latex. Even in the case of ComparativeExample 1, enlargement by aggregation is caused to progress. However,the restabilizing effect by the cationic surfactant upon lowering the pHof the system is not developed. Therefore, when sodium hydroxide wasadded to enhance the pH of the system, thereby attempting to stabilizethe system, solids are deposited due to shearing force by the stirring,resulting in a failure to obtain a stable enlarged latex.

Comparative Example 2 shows an example where it was intended to obtain alatex having the same particle diameter as in Example 1. It takes a verylong time to conduct the polymerization. Therefore, the example is poorin productivity and not an economical process. Comparative Example 3shows an example where Example (A-2) described in Japanese PatentApplication Laid-Open No. 71603/1998, in which enlargement of particlediameter to 260 nm was achieved, was simulated with no cationicsurfactant used, and the part of phosphoric acid added was increased soas to achieve a greater particle diameter. However, the latex wassolidified as a whole, resulting in a failure to obtain an enlargedlatex.

Example 4

1. Graft Polymerization:

After a pressure container equipped with a stirrer was charged with

Enlarged Latex (B-1) 75 parts (in terms of solids) potassium oleate 0.3parts tetrasodium pyrophosphate 0.005 parts heated to 60° C. and purgedwith nitrogen, styrene 12.5 parts methyl methacrylate 12.5 partsdiisopropylbenzene hydroperoxide 0.1 parts sodiumformaldehydesulfoxylate 0.05 parts

were added over 1 hour. The contents were then held for 5 hours. As aresult, Graft Copolymer Latex (C-4) having a volume average particlediameter of 480 nm and a Dv/Dn of 1.35 was obtained.

After 0.5 parts of butylated hydroxytoluene (BHT) were added to thisGraft Copolymer Latex (C-4), coagulation was conducted with 0.5%hydrochloric acid, and the resultant coagulum was washed with water,dehydrated and dried to obtain a powdered Graft Copolymer (D-4). Therefractive index of the polymer (B-1) to be grafted in this GraftCopolymer (D-4) was 1.539, and the refractive index of Graft Copolymer(D-4) was 1.539.

Example 5

Graft Copolymer Latex (C-5) was obtained and Graft Copolymer (D-5) wasrecovered in the same manner as in Example 4 except that

sodium formaldehydesulfoxylate 3.3 parts (5% aqueous solution) hydrogenperoxide (5% aqueous solution) 1.8 parts

were used upon the enlargement of Latex (A-1) used in obtaining thegraft copolymer of Example 4 by aggregation. Graft Copolymer Latex (C-5)had a volume average particle diameter of 205 nm and a Dv/Dn of 1.37.The refractive index of the polymer (B-5) to be grafted in this GraftCopolymer (D-5) was 1.539, and the refractive index of Graft Copolymer(D-5) was 1.539.

Comparative Example 4

1. Enlargement by Aggregation:

While holding the Latex (a-3) obtained in Comparative Example 3 at 60°C.,

disodium dodecyl phenyl ether 0.05 parts disulfonate was added, andhydrochloric acid 56 parts (0.15% aqueous solution) was then added toconduct enlargement by aggregation, and sodium hydroxide (1% aqueoussolution) 10 parts

was then added to obtain Enlarged Latex (b-4). Enlarged Latex (b-4) hada volume average particle diameter of 200 nm and a Dv/Dn of 1.90 and wasmechanically stable. Incidentally, the sum total of deposits andattachments to the polymerizer was 0.10% based on the charged monomers.

2. Graft Polymerization:

Enlarged Latex (b-4) was used to conduct graft polymerization in thesame manner as in Example 4, thereby obtaining Graft Copolymer Latex(c-4) having a volume average particle diameter of 205 nm and a Dv/Dn of1.85, and powdered Graft Copolymer (d-4) was obtained in the same manneras in Example 4. The refractive index of the polymer (b-4) to be graftedin Graft Copolymer (d-4) was 1.539, and the refractive index of GraftCopolymer (d-4) was 1.539.

<Physical Properties of Resin Composition>

Each of the graft copolymers obtained in Examples 4 and 5 andComparative Example 4 was used to prepare a resin composition with athermoplastic resin (MS resin) in accordance with the followingformulation, thereby determining its physical properties. The refractiveindex of the MS resin used was 1.540.

Graft copolymer 30 parts MS resin (Denka TX-100, product of 70 partsDenki Kagaku Kogyo Kabushiki Kaisha)

The resin composition was pelletized by means of a twin-screw conicalextruder having a diameter of 20 mm manufactured by Toyo SeikiSeisaku-Sho, Ltd., and the thus-obtained pellets were molded into aspecimen for impact test having a thickness of 6 mm by means of aninjection molding machine IS-80 manufactured by Toshiba Machine Co.,Ltd. The impact test was performed at 23° C. The results are shown inTable 1.

TABLE 1 Physical properties of thermoplastic resin compositions (MSresin-based) Graft Impact strength Transmittance for Haze copolymer[kJ/m²] parallel rays [%] [%] Ex. 4 D-4 17.8 85 3.2 Ex. 5 D-5 5.8 90 1.7Comp. d-4 3.2 89 2.0 Ex. 4

As shown in Table 1, the resin composition comprising the graftcopolymer (Example 4) enlarged in particle diameter according to thepresent invention is achieved in a great improvement on impact strengthcompared with the resin composition comprising the graft copolymer(Comparative Example 4) prepared by using the ordinary technique forenlargement by aggregation though the transparency is slightlydeteriorated. The resin composition making use of the graft copolymer(Example 5) according to the present invention, which has the sameparticle diameter as that in Comparative Example 4 and a narrowerparticle diameter distribution, is improved in balance between impactstrength and transparency compared with the resin composition making useof the graft copolymer of Comparative Example 4.

Example 6

1. Graft Polymerization:

After a pressure container equipped with a stirrer was charged with

Enlarged Latex (B-3) 75 parts (in terms of solids) potassium oleate 0.3parts tetrasodium pyrophosphate 0.005 parts heated to 60° C. and purgedwith nitrogen, methyl methacrylate 12.5 parts butyl acrylate 2.5 partst-butyl hydroperoxide 0.2 parts sodium formaldehydesulfoxylate 0.2 partswere added over 1 hour. After the contents were then held for 3 hours,styrene 10 parts t-butyl hydroperoxide 0.1 parts sodiumformaldehydesulfoxylate 0.1 parts

were added over 1 hour. The contents were then held for 5 hours toobtain Graft Copolymer Latex (C-6) having a volume average particlediameter of 265 nm and a Dv/Dn of 1.37. Powdered Graft Copolymer (D-6)was obtained from this Graft Copolymer Latex (C-6) in the same manner asin Example 4.

Comparative Example 5

1. Preparation of Latex:

After a pressure container equipped with a stirrer was charged with

tetrasodium pyrophosphate 0.1 parts ferrous sulfate 0.005 parts disodiumethylenediaminetetraacetate 0.008 parts sodium formaldehydesulfoxylate0.05 parts disodium dodecyl phenyl ether 0.02 parts disulfonatepotassium oleate 0.37 parts distilled water 200 parts and purged withnitrogen, diisopropylbenzene hydroperoxide 0.1 parts butadiene 100 parts

were added. The contents were then held at 70° C. for 15 hours to obtainLatex (a-5) having a conversion of 98% and a volume average particlediameter of 95 nm.

2. Enlargement by Aggregation:

While holding the Latex (a-5) obtained above at 60° C.,

hydrochloric acid 60 parts (0.15% aqueous solution) was added to conductenlargement by aggregation, and sodium hydroxide (1% aqueous solution)10.7 parts

was then added to obtain Enlarged Latex (b-5). Enlarged Latex (b-5) hada volume average particle diameter of 220 nm and a Dv/Dn of 1.87 and wasmechanically stable. Incidentally, the sum total of deposits andattachments to the polymerizer was 0.12% based on the charged monomer.

3. Graft Polymerization:

Enlarged Latex (b-5) was used to conduct graft polymerization in thesame manner as in Example 6, thereby obtaining Graft Copolymer Latex(c-5) having a volume average particle diameter of 225 nm and a Dv/Dn of1.85, and powdered Graft Copolymer (d-5) was then obtained in the samemanner as in Example 4.

<Physical Properties of Resin Composition>

Each of the graft copolymers obtained in Example 6 and ComparativeExample 5 was used to prepare a resin composition with a thermoplasticresin (vinyl chloride resin) in accordance with the followingformulation, thereby determining its physical properties. Morespecifically, the following blending components were provided andcharged into a Henschel mixer and heated to 115° C. with stirring toobtain a resin composition evenly mixed.

Graft copolymer 9 parts Vinyl chloride resin (“S9008”, 91 parts productof Kureha Kagaku Kogyo K.K.) Processing aid (“K130P”, product of 1.0part Kureha Kagaku Kogyo K.K.) Octyltin mercaptide (“KS2000A”, 1.8 patsproduct of Kyodo Chemical Co., Ltd.) Calcium stearate (“Ca-St”, product0.5 parts of Nitto Kasei Co., Ltd.) Ester-based lubricant (“SL-02”, 0.6parts product of Riken Vitamin Co., Ltd.) Titanium oxide (“DP-3T-55”,product 0.1 part of Resino Color Industry Co., Ltd.)

The resin composition thus obtained was pelletized by means of atwin-screw conical extruder having a diameter of 20 mm manufactured byToyo Seiki Seisaku-Sho, Ltd., and the thus-obtained pellets were moldedinto a specimen for impact test having a thickness of 3 mm by means ofan injection molding machine IS-80 manufactured by Toshiba Machine Co.,Ltd. The impact test was performed at 23° C. and −10° C. The results areshown in Table 2.

TABLE 2 Physical properties of thermoplastic resin compositions(PVC-based) Graft Impact strength (23° C.) Impact strength copolymer[kJ/m²] (−10° C.) [kJ/m²] Ex. 6 D-6 116 41.0 Comp. d-5 116 18.9 Ex. 5

As shown in Table 2, the resin composition comprising the graftcopolymer (Example 6) according to the present invention is achieved ina great improvement on impact strength at the low temperature comparedwith the resin composition comprising the graft copolymer (ComparativeExample 5) prepared by using the ordinary technique for enlargement byaggregation though the same impact resistance is exhibited at 23° C.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided a process forpreparing an enlarged latex, which is economical and high inproductivity and permits enlarging a latex while retaining the stabilityof the latex. According to the preparation process of an enlarged latexaccording to the present invention, the stability of the latex is notimpaired even when a polymerizable monomer is graft polymerized in thepresence of the enlarged latex.

According to the present invention, there are also provided enlargedlatices having such excellent various properties, a process forpreparing a graft copolymer by polymerizing a polymerizable monomer inthe presence of such an enlarged latex, resin compositions containingsaid graft copolymer and a thermoplastic resin, and graft copolymershaving a great particle diameter and an even particle diameterdistribution. The resin compositions according to the present inventionare economically improved in physical properties (particularly, impactresistance). Accordingly, the present invention can be applied tovarious fields in industries of which a latex having a great particlediameter and a graft copolymer having a great particle diameter arerequired.

What is claimed is:
 1. A process for preparing an enlarged latex byenlarging a latex by aggregation, which comprises (1) causing (a) ananionic surfactant and (b) at least one surfactant selected from thegroup consisting of a cationic surfactant and an amphoteric surfactantto exist in the latex, (2) adding, as an aggregating and enlargingagent, at least one selected from the group consisting of: (i) aninorganic acid, (ii) an organic acid, (iii) a substance which forms anacid in water, (iv) a combination of at least two substances which arereacted with each other to form an acid, and (v) a substance which formsan acid by exposure to active rays to the latex in the presence of thesesurfactants, and (3) causing the acid derived from the aggregating andenlarging agent to act on the latex, thereby enlarging the particlediameter of the latex.
 2. The preparation process according to claim 1,wherein the latex is a (co)polymer latex of diene monomer(s),(co)polymer latex of vinyl monomer(s) or copolymer latex of a dienemonomer with a vinyl monomer.
 3. The preparation process according toclaim 1, wherein (a) the anionic surfactant and (b) at least onesurfactant selected from the group consisting of a cationic surfactantand an amphoteric surfactant are used as surfactants in the step (1) toemulsion polymerize the monomer(s), thereby causing these surfactants(a) and (b) to exist in the latex.
 4. The preparation process accordingto claim 1, wherein the anionic surfactant is a carboxylate type anionicsurfactant.
 5. The preparation process according to claim 1, wherein thecationic surfactant is a quaternary ammonium salt having at least onealkyl group, a primary to tertiary amine salt having at least one alkylgroup, an alkylphosphonium salt or an alkylsulfonium salt.
 6. Thepreparation process according to claim 1, wherein the amphotericsurfactant is a betaine, a carboxylic acid type amphoteric surfactantcontaining a betaine, a sulfate type amphoteric surfactant or a sulfonicacid type amphoteric surfactant.
 7. The preparation process according toclaim 1, wherein in the step (1), (b) at least one surfactant selectedfrom the group consisting of the cationic surfactant and the amphotericsurfactant is caused to exist in the latex in a proportion of 0.01 to100 mol per 100 mol of (a) the anionic surfactant.
 8. The preparationprocess according to claim 1, wherein in the step (1), the surfactants(a) and (b) are caused to exist in a proportion of 0.1 to 5 parts byweight in total per 100 parts by weight of the (co)polymer component inthe latex or the monomer(s) forming the (co)polymer component.
 9. Thepreparation process according to claim 1, wherein in the step (2), acombination of two substances which undergo a redox reaction to form anacid is used as (iv) the combination of at least two substances whichare reacted with each other to form an acid.
 10. The preparation processaccording to claim 9, wherein the combination of two substances whichundergo a redox reaction to form an acid is a combination of hydrogenperoxide with a formaldehydesulfoxylate.
 11. The preparation processaccording to claim 1, wherein in the step (3), the latex is enlarged byBrownian aggregation without conducting stirring of the latex.
 12. Thepreparation process according to claim 1, wherein an enlarged latexhaving a volume average particle diameter of at least 150 nm is providedby the enlargement by aggregation.
 13. The preparation process accordingto claim 1, wherein an enlarged latex having a particle diameterdistribution of 1.2 to 1.8, represented by a ratio (Dv/Dn) of the volumeaverage particle diameter (Dv) to the number average particle diameter(Dn) is provided.
 14. An enlarged latex obtained by the preparationprocess according to claim
 1. 15. A process for preparing a graftcopolymer, which comprises polymerizing a polymerizable monomer in thepresence of an enlarged latex, wherein the enlarged latex is obtained bya process comprising (1) causing (a) an anionic surfactant and (b) atleast one surfactant selected from the group consisting of a cationicsurfactant and an amphoteric surfactant to exist in a latex, (2) adding,as an aggregating and enlarging agent, at least one selected from thegroup consisting of: (i) an inorganic acid, (ii) an organic acid, (iii)a substance which forms an acid in water, (iv) a combination of at leasttwo substances which are reacted with each other to form an acid, and(v) a substance which forms an acid by exposure to active rays to thelatex in the presence of these surfactants, and (3) causing the acidderived from the aggregating and enlarging agent to act on the latex,thereby enlarging the particle diameter of the latex.
 16. Thepreparation process according to claim 15, wherein the polymerizablemonomer is a vinyl monomer.
 17. The preparation process according toclaim 15, wherein 95 to 5 wt. % of the vinyl monomer is polymerized with5 to 95 wt. %, in terms of solids, of the enlarged latex.
 18. Thepreparation process according to claim 15, wherein a graft copolymerhaving a particle diameter distribution of 1.2 to 1.8, represented by aratio (Dv/Dn) of the volume average particle diameter (Dv) to the numberaverage particle diameter (Dn), is obtained.
 19. The preparation processaccording to claim 15, wherein a difference in refractive index betweenparticles of the enlarged latex and the graft copolymer formed is atmost 0.02.
 20. The preparation process according to claim 15, wherein agraft copolymer having a volume average particle diameter of at least150 nm is obtained.
 21. A graft copolymer obtained by the preparationprocess according to claim
 15. 22. A resin composition comprising thegraft copolymer according to claim 21 and a thermoplastic resin.
 23. Theresin composition according to claim 22, which comprises 0.1 to 99.9 wt.% of the graft copolymer and 99.9 to 0.1 wt. % of the thermoplasticresin.
 24. The resin composition according to claim 22, wherein thethermoplastic resin is polystyrene, high impact polystyrene, acrylicresin, methyl methacrylate-styrene resin, vinyl chloride resin,chlorinated vinyl chloride resin, acrylonitrile-styrene resin,acrylonitrile-butadiene-styrene resin, thermoplastic polyester resin,polycarbonate resin or a mixture thereof.
 25. A graft copolymercontaining an anionic surfactant and at least one surfactant selectedfrom the group consisting of a cationic surfactant and an amphotericsurfactant and having a ratio (Dv/Dn) of the volume average particlediameter (Dv) to the number average particle diameter (Dn) of 1.2 to1.8.
 26. The graft copolymer according to claim 25, wherein the volumeaverage particle diameter (Dv) is at least 150 nm.
 27. A resincomposition comprising the graft copolymer according to claim 25 and athermoplastic resin.